(2014/07/24 18:30), Shigeru Hanada wrote:
My coworker Takaaki EITOKU reviewed the patch, and here are some
comments from him.

Thank you for the review, Eitoku-san!

2014-07-08 16:07 GMT+09:00 Etsuro Fujita <fujita.ets...@lab.ntt.co.jp>:
In the patch postgresPlanForeignModify has been modified so that if, in
addition to the above condition, the followings are satisfied, then the
ForeignScan and ModifyTable node will work that way.

  - There are no local BEFORE/AFTER triggers.

The reason the check ignores INSTEAD OF triggers here is that INSTEAD
OF trigger would prevent executing UPDATE/DELETE statement against a
foreign tables at all, right?

I'm not sure that I understand your question correctly, but the reason for that is because foreign tables cannot have INSTEAD OF triggers.

  - In UPDATE it's safe to evaluate expressions to assign to the target
columns on the remote server.

IIUC, in addition to target expressions, whole WHERE clause should be
safe to be pushed-down.  Though that is obviously required, but
mentioning that in documents and source comments would help users and

OK, I'll add the comments and documentation notes.

(I intentionaly didn't mention that because I think the comment for postgresPlanForeignModify has already said the equivalent condition, ie, "there are no local conditions", which means all conditions are executed remotely.)

We found that this patch speeds up DELETE case remarkably, as you
describe above, but we saw only less than 2x speed on UPDATE cases.
Do you have any numbers of UPDATE cases?

Here is the result for an UPDATE case.

>> On remote side:
>> postgres=# create table t (id serial primary key, inserted timestamp
>> default clock_timestamp(), data text);
>> postgres=# insert into t(data) select random() from generate_series(0,
>> 99999);
>> INSERT 0 100000
>> postgres=# vacuum t;
>> On local side:
>> postgres=# create foreign table ft (id integer, inserted timestamp, data
>> text) server myserver options (table_name 't');

postgres=# explain analyze verbose update ft set data = 'notsolongtext' where id < 10000;
                                                       QUERY PLAN
Update on public.ft (cost=100.00..147.38 rows=650 width=18) (actual time=1463.944..1463.944 rows=0 loops=1)
   Remote SQL: UPDATE public.t SET data = $2 WHERE ctid = $1
-> Foreign Scan on public.ft (cost=100.00..147.38 rows=650 width=18) (actual time=2.069..83.220 rows=9999 loops=1)
         Output: id, inserted, 'notsolongtext'::text, ctid
Remote SQL: SELECT id, inserted, ctid FROM public.t WHERE ((id < 10000)) FOR UPDATE
 Planning time: 0.355 ms
 Execution time: 1470.032 ms
(7 rows)

postgres=# explain analyze verbose update ft set data = 'notsolongtext' where id < 10000;
                                                     QUERY PLAN
Update on public.ft (cost=100.00..147.38 rows=650 width=18) (actual time=0.005..0.005 rows=0 loops=1) -> Foreign Scan on public.ft (cost=100.00..147.38 rows=650 width=18) (actual time=0.001..0.001 rows=0 loops=1)
         Output: id, inserted, data, ctid
Remote SQL: UPDATE public.t SET data = 'notsolongtext'::text WHERE ((id < 10000))
 Planning time: 0.197 ms
 Execution time: 61.519 ms
(6 rows)

I think that the precise effect of this optimization for DELETE/UPDATE would depend on eg, data, queries (inc. w/ or w/o RETRUNING clauses) and server/network performance. Could you tell me these information about the UPDATE evaluation?

Some more random thoughts:

* Naming of new behavior
You named this optimization "Direct Update", but I'm not sure that
this is intuitive enough to express this behavior.  I would like to
hear opinions of native speakers.


* Macros for # of fdw_private elements
In postgres_fdw.c you defined MaxFdwScanFdwPrivateLength and
MinFdwScanFdwPrivateLength for determining the mode, but number of
fdw_private elements is not a ranged value but an enum value (I mean
that fdw_private holds 3 or 7, not 4~6, so Max and Min seems
inappropriate for prefix.  IMO those macros should be named so that
the names represent the behavior, or define a macro to determine the
mode, say IS_SHIPPABLE(foo).

OK, Will fix.

* Comparison of Macros
Comparison against MaxFdwScanFdwPrivateLength and
MinFdwScanFdwPrivateLength should be == instead of >= or <= to detect
unexpected value.  Adding assert macro seems good too.

Will fix this too.


Best regards,
Etsuro Fujita

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